1. Field of the Invention
The present invention relates to a light emitting diode (LED) having gallium arsenide-phosphide (GaAsP) layer as an active layer, and more particularly to the inexpensive LED with an enhanced optical output.
2. Description of the Prior Art
A carrier injection type semiconductor light emitting diode has, as its great characteristic merit, a long lifetime and a fast response. However, since an optical output of the general LED has been small due to a p-n homo junction structure, its application has been limited In recent years, however, owing to improvements in the internal structure of the LED, more specifically in a multi-layer structure thereof, it has made a great stride in getting a high optical output, and so, there has appeared a possibility that the application area of the LED may be remarkably broadened. As examples of the new application, one can enumerate tail lamps of motor cars, traffic signals and neon signs.
The recently developed high optical output LED has a p-n hetero junction structure consisting of an active layer of gallium-aluminium arsenide (GaAlAs) and a window layer of GaAlAs having a larger band gap than that of the active layer and thereby suppressing absorption of output light (red) emitted from a light emitting region of the active layer through the window layer. More specifically, the GaAlAs window layer has a larger aluminium content than that of the active layer and is used as an electrode contact layer on an exit side of the output light. In addition to the above LED structure resorting a so-called window effect, a so-called double heterostructure has been proposed in order to realize a still further high optical output. In the known double heterostructure, an active layer of GaAlAs is sandwiched between a pair of confining layers of GaAlAs having large aluminium content than that of the active layer, and thereby confining injected carriers into the active layer. One of the confining layers also acts as a window layer by forming p-n hetero junction against the active layer.
However, such GaAlAs-LED has following problems in manufacture due to the fact that aluminium is extremely liable to be oxidized.
Firstly, as one problem in the step of crystal growth, a severe counter-measure for reducing residual oxygen (moisture) gas is necessitated so that aluminium may not be oxidized. Normally in epitaxial growth, a liquid phase growth process is employed, and the liquid phase growth process requires a great many severe attentions such as deoxidation of various jigs within a growth reaction tube as a matter of course, removal of residual oxygen in a carrier gas, prevention of minute air leakage, prevention of contamination of air upon charging and discharging a crystal, and preliminary baking of a raw material before growth. Such counter-measures would naturally result in reduction of a productivity, lowering of a yield and rise of a cost.
As a second shortcoming, even if growth of a high-quality crystal were to become possible, special counter-measures which are caused by the inclusion of easily oxidizable aluminium are also necessary in the steps before completing the LED. In any one of a high-concentration diffusion process for ohmic contact, an electrode applying process, a passivation process, a pelletizing process, a mount sealing process, etc., mixing of moisture and oxygen must be completely suppressed, and for that counter-measure a lot of expense is necessary.
Recently for the purpose of improving a mass-productivity of epitaxial growth, developments of an organic metal vapor phase epitaxial growth process have been made actively in place of the liquid phas epitaxial growth process. However, the above-described first shortcoming still remains and makes manufacture of a high-quality crystal difficult. From the above-mentioned view point, it has been desired to develop an LED with an enhanced optical output and a low manufacturing cost by eliminating aluminium content.
On the other hand, an LED using an active layer of GaAsP has been already put to practical use. This GaAsP-LED is manufactured starting from an epitaxial wafer in which a GaAsP layer is grown on a GaAs or GaP substrate through a halogen transport vapor epitaxial growth process, and it emits red to green light by changing the contents of phosphide. Since aluminium is not contained in the GaAsP-LED, it is easy to form a high-quality epitaxial layer through a vapor phase growth process. This GaAsP-LED has a merit that the manufacturing cost of the epitaxial layer can be suppressed extremely due to the fact that the vapor phase growth process has a high productivity.